GC: introduce back old GC
ECL had its own portable GC (not bdwgc). For portability reasons we should fix its builds and reintroduce it. This could make some experiments possible, like concurrency[1] etc. Commit removing old GBC is listed in the new documentation[2].
[1] https://blog.pusher.com/golangs-real-time-gc-in-theory-and-practice/
[2] https://common-lisp.net/project/ecl/static/ecldoc/Developer_0027s-guide.html#Removed-features